The present invention relates to methods and materials for forming near net-shape metal and metal matrix composite components. More specifically, the invention relates to methods of infiltration casting using investment molds to produce near net-shape metal and metal matrix composite components.
Infiltration casting, and in particular pressure infiltration casting, is a widely used method for manufacturing net-shape metal and metal matrix composite (MMC) parts. Methods for MMC parts typically involve placing a ceramic preform material into a machined mold cavity, placing the mold into a mold vessel, then evacuating the mold vessel to create a reduced pressure in the mold cavity. Subsequently, a molten metal is introduced on top of or around the mold cavity to seal it from the surrounding atmosphere, then forcing the molten metal with external pressure into the mold cavity and ceramic preform. After the molten metal solidifies, a net-shape MMC part is mechanically extracted from the mold.
The molds for pressure infiltration casting of ceramic preforms usually are machined graphite or steel. Machining graphite or steel molds into complex or intricate shapes is an extremely expensive and laborious process. Moreover, both steel and graphite molds can be used only a limited number of times before being rendered unuseable due to surface wear and erosion. In addition, the high cost of quality graphite drives production costs even higher. Therefore, the mold-making stage of an infiltration casting manufacturing process tends to make MMC production technology economically prohibitive and inflexible.
An alternative to using expensive mold tooling is a xe2x80x9cmoldlessxe2x80x9d process, such as sand or investment casting techniques, where a preform is supported by sand or an investment material which encases the preform. However, these technologies typically cannot be applied directly to pressure casting. Since sand and investment materials used in these processes usually are porous, upon application of external pressure to a molten metal, the sand or investment mold material usually is infiltrated along with the ceramic preform. Consequently, a continuous MMC part and investment material body is formed, making it nearly impossible to extract a net-shape MMC part.
To avoid infiltration of the investment compound, a refractory or gas impermeable shell mold may be used. However, castable refractory or gas impermeable shell molds typically are hard, making removal of the cast part from the investment mold difficult without damaging the MMC part. This alternative is complicated further when a complex component having detailed and/or fine features is the ultimate goal. A further concern is the cost of mold materials themselves, which is appreciable. Moreover, state-of-the-art mold casting technologies do not lend themselves to recycling the mold materials, thereby increasing the cost of the resulting metal or MMC parts.
Although a technique has been described whereby a self-supporting preform is encased by an investment mold material, the method involves sintering the preform during curing of the investment mold. See, e.g., U.S. Pat. Nos. 5,234,045 or 5,297,609 to Cook et al. Sintering of the preform ceramic particulates while curing the investment mold material may be unnecessary and/or undesirable, e.g., requiring excessively high temperatures, and/or changing the porosity of the preform, e.g., reducing the porosity which may affect removal of fugitive materials and/or water of hydration, and/or modifying the pattern of the net-shape mold cavity.
Although commercially available refractory cement materials have certain desirable properties, e.g., low coefficient of thermal expansion (CTE), low shrinkage during the high temperature of the casting process, and a smooth surface, most are not useful in pressure infiltration processes since these refractory cement materials are too porous and are infiltrated by molten metals during pressure infiltration casting. In addition, these materials often are sintered during high temperature casting processes.
Accordingly, there is a need for a simple and reliable pressure infiltration casting method similar to conventional sand or investment casting processes, which allows easy extraction of near net-shape metal or MMC components by preventing infiltration of a molten metal under external pressure into the investment mold.
It has been discovered that near net-shape metal or metal matrix composite (MMC) components can readily be produced without expensive machined tools by using investment casting materials and molds of the invention. When combined with a net-shape preform or fugitive mold pattern, the investment casting methods of the invention are TOOL-LESS MOLD(trademark) casting processes. By eliminating the need for expensive machined tools, a cost effective manufacturing process is realized which enables rapid prototype production as well as an economically feasible way to produce metal or MMC components in mass. Such methodology is widely applicable, and is especially suitable for automotive, engine, and aerospace applications where the demands for complex shapes and low manufacturing costs are high. Methods of the invention also include producing near net-shape metal or MMC components using modified conventional investment mold materials such as ceramic cements.
In a broad embodiment directed to MMC parts, a method of the invention for forming a MMC component includes the steps of (i) providing a net-shape preform of the MMC component to be cast; (ii) forming an investment mold of a refractory material around the preform; (iii) infiltrating the preform with a metal, to the exclusion of the investment mold, using an infiltration casting process, e.g., the Advanced Pressure Infiltration Casting (APIC(trademark)) process; (iv) solidifying the metal; and (v) removing the investment mold material to provide the final MMC part.
In other embodiments, a net-shape pattern, adjacent to a pre-gate, is positioned in a container to which is added a refractory material. The powdered refractory material, which often includes a vehicle, is disposed around the pattern, e.g., a preform, and at least a portion of the pre-gate. Subsequently, in preferred embodiments, the vehicle is removed from the container to create a dried, packed particulate refractory material, i.e., an investment mold which defines a mold cavity containing a preform, if present. In addition, the pre-gate is removed to form a gate which is in fluid communication with the pattern. A molten metal is introduced into the container and forced into the mold cavity (and a preform) through the gate, but not into the investment mold. After infiltration of the mold cavity (and preform) is complete, the molten metal is cooled and the investment mold is removed to afford the metal or MMC part having the shape of the pattern.
In another embodiment of the invention, commercially available castable refractory cement materials, e.g., Ceredyne CASTABLE 220 (THERMO-SIL CASTABLE 220) (Ceredyne Inc., Scottdale, Ga.), may be modified to produce investment molds useful in the practice of the invention. The addition of a non-corrosive liquid soluble refractory material such as magnesium oxide to commercially available castable refractory cements can overcome their deficiencies for use as an investment mold material. Magnesium oxide is soluble in non-corrosive solvents such as water, and is stable to high temperature applications often encountered during pressure infiltration casting, i.e., it does not decompose at high temperatures and does not interact with molten metals. As a result, the modified ceramic cement may be used as an investment mold material since it readily can be separated from the cast part.
Another aspect of the invention is a reusable investment mold which preferably is formed from a slurry of a vehicle and a refractory material. The reusable investment mold of the invention particularly is suited for practicing methods of the invention. Preferably, the investment mold is made of fine particulates of refractory materials which are closely packed to produce a dense, low porosity investment mold. The vehicle preferably is water, water with an organic or inorganic additive, or an organic solvent such as an alcohol. The refractory materials preferably are alumina, silica, magnesia, or feldspar. The average particle size of the refractory materials preferably are on the order of about one micron or less.
Reference to the figures herein is intended to provide a better understanding of the methods and apparatus of the invention but are not intended to limit the scope of the invention to the specifically depicted embodiments. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Like reference characters in the respective figures typically indicate corresponding parts.
It should be understood that the order of the steps of the methods of the invention is immaterial so long as the invention remains operable, i.e., e.g., a net-shape pattern of the final cast part must be provided prior to formation of the investment mold. In addition, an investment mold, optionally containing a preform, must be created prior to infiltration of a molten infiltrant into the mold cavity of the investment mold. Moreover, two or more steps may be conducted simultaneously.